Air spraying methods have conventionally been used in the art to coat articles formed of a crystalline thermoplastic resin. However, the coating deposition efficiency for such air spray methods is as low as between about 20 to 50%, thereby inevitably increasing the cost of the coated articles. For this reason, electrostatic coating has attracted attention as an alternative method which can achieve high paint coating deposition efficiency. However, electrostatic coating has typically been utilized for coating electrically conductive metals, and thus has not been employed as a means to coat articles formed of a material having poor electrical conductivity (e.g. resins).
It has however been proposed to apply to resin articles an undercoat of a conductive agent composed mainly of a cationic surfactant to the surface of plastic articles so as to achieve electrical surface conductivity of between 10.sup.3 to 10.sup.9 .OMEGA.. Thereafter, the plastic article may be subjected to electrostatic coating. However, since this proposed method requires the use of a hydrophilic solvent as the conductive agent, the surface of the molding may attract moisture to the extent that pinholes and blisters are formed during drying of the top coating. Furthermore, although such an undercoat can be applied to an amorphous thermoplastic resin, its application to a thermoplastic resin causes an adverse effect on the adhesive strength of the coating.
In addition to the above-mentioned prior proposals, it has also been suggested to employ a primer paint containing a conductive filler (rather than using a conductive agent for imparting conductivity to the paint). However, significant economic disadvantages are presented by use of a conductive primer, including the necessity to resort to the inefficient air spray coating method when applying the primer, an increase in the number of coating types, difficulties when applying a uniform coating to complex moldings, and the necessity that the coating step be accomplished manually (rather than via automated procedures). In addition, further difficulties are encountered when a thin film coating is applied resulting in film adhesion which is usually less than satisfactory.
By way of the present invention, electrostatic coating methods are proposed which exhibit high coating deposition efficiency in the electrostatic surface coating of crystalline thermoplastic resins. The resulting coating exhibits excellent adhesive strength. Thus, according to the present invention, a film coating having high adhesive strength can uniformly be obtained with high coating deposition efficiency by roughening the surface of a molded article of a crystalline thermoplastic resin having electrical conductivity imparted thereto by the addition of a conductive filler. By roughening the molded article, a thin polycrystalline resin skin layer normally covering the surface of the article is removed, thereby enhancing the article's electrical conductivity by exposing the article's conductive filler on the surface of the molding. At the same time, the paint adhesion properties are improved by means of such surface-roughening through an anchoring effect onto the surface of the molding.